1. Field of the Invention
The instant disclosure relates to a magnetic lens; in particular, to a wide range tunable magnetic lens.
2. Description of Related Art
In recent years, the demand for observation of biological samples has increased due to the development of biotechnology. The electron microscope has been used to observe the biological samples, but surface charge accumulation on samples occurs due to the high accelerating voltage. At present, a method of decreasing the accelerating voltage for electrons has been adopted, in which the accelerating voltage is typically ranged from 0.5 kV to 5 kV. In general, the desktop electron microscope utilizes a permanent magnetic lens to replace the conventional coil magnetic lens, but will simultaneously lose the tunable ability under different accelerating voltages.
The focusing power of a magnetic field is proportional to the root of the accelerating voltage. As in FIG. 1, the horizontal axis shows the accelerating voltage (1 kV per unit), and the vertical axis shows the magnetic excitation increment. The magnetic excitation increment corresponding to the accelerating voltage increasing from 1 kV to 2 kV is about three times of the magnetic excitation increment corresponding to the accelerating voltage increasing from 14 kV to 15 kV.